This started as a response to a comment on another blog, but grew until it wasn't really appropriate as a comment any longer.
I think that saying anyone considers their own lifestyle best is something of a tautology. Of course I think my lifestyle is best, that's why I live it. If I thought another was superior, I would change; in fact, when I find aspects of my lifestyle that are lacking in relation to others I make an effort to change. We realise that we are none of us perfect, but while thinking that perhaps we ought to eat better, exercise more, floss more often, etc., we do all of us think that our lifestyle is best considered in broad strokes. I can (and do) consider many other cultures inferior while at the same time respecting their members as humans worth my kindness. And indeed, given the knowledge of my own culture's superiority, isn't a kindness to urge others to adopt it insofar as they are able?
The key is how one goes about it, obviously. As a conservative (with a lower-case "c"), because I think that generally the traditional is superior to the innovative, I am an advocate of life as it is lived and was lived rather than urging that we change from what we have known to some promised utopia. Someone once described, partially in jest, a liberal as a man too broad-minded to take his own side in an argument. In this instance, because of the dangers of using a position of strength to impose one's ideas on others, Americans are chided by some to not advocate their culture at all.
In much the same way, Christians are asked not to do any proselytising either. But to be a Christian is to proselytise; to do otherwise is to ignore a direct command of Jesus to His disciples. And if we think we have found Truth in being Christians, not to share this with others is to willingly condemn them to blindness in this life and damnation in the next. This is all so obvious that I wonder anyone can fail to understand it. I've noted before that it is obvious and normal to always think you're right, but the height of arrogance to think you're always right.